Si/Ge nanodot superlattices for Si-based photovoltaics

We have grown Si/Ge nanodot superlattices via low-pressure chemical vapor deposition in order to analyze their performance as thin-film solar cells. Self-assembled Ge nanodots are included in the base region in order to boost absorption of near-infrared photons and to increase short-circuit current density, J_sc. At a relatively low dot density of 5.5x10⁹ cm^-2, both 20- and 40-period cells exhibited a fill factor of 70% and opencircuit voltage (V_oc) of 0.51V, closely matching previously reported devices grown by molecular beam epitaxy. The 20- and 40-period cells had similar spectral responsivity for λ=400-550 nm, but the thicker base of the 40-period cell enabled it to attain higher responsivity for wavelengths in the range of 550-900 nm. When we increased the dot density by 55% while holding the number of periods at 40, V_oc dropped significantly due to a combination of lower bandgap and higher dislocation density. Work is in progress to integrate such SiGe-nano-materials based PV devices with ultra-thin Si PV, to obtain higher efficiencies as well as minimize the use of Si.